Compounds capable of absorbing and/or emitting light are applicable in a variety of optical and photoelectric devices, including but not limited to photo-absorbing devices such as solar- and light-sensitive devices, organic light-emitting diodes (OLEDs), light-emitting devices, devices capable of absorbing and emitting light and biological markers. In order to find organic and organometallic materials used in optical and photoelectric devices, many researchers have done associated studies. Generally, research in this area aims to find an applicable organic phosphorescent material. The primary goal in the fields of display and illumination fields is to improve stability of the materials and the devices, spectral characteristics and luminescent efficiency.
Despite significant advances in research devoted to photoelectric materials (e.g., red and green phosphorescent organometallic materials used as phosphors, and blue organometallic material used as fluorescent material), and success have been made in applications such as organic light emitting diode (OLED) illumination and advanced displays, there are still disadvantages in large-size display device applications, such as short luminescence lifetime, high heat release and low efficiency in actual applications. And these disadvantages can be proved by the marketed LG OLED television. Particularly, there is no effective way to solve the problem of the burn-in of TV channel logo after long-term fixed broadcast. From the prospective of application, in order to solve these problems, it is necessary to greatly improve the stability of materials and devices, and to set higher requirements on material design.
For the application of a small-sized OLED display (mainly used for mobile phone display) in which good effects are obtained, a blue fluorescent material is currently used, which has a low theoretical efficiency and does not belongs to a same system with other materials, thereby increasing the processing difficulty and cost. Therefore, improving the overall performance is the most important to solve the problem of blue phosphorescent materials and achieve stable light-emitting devices. Literature: Sinheui Kim, Hye Jin Bae, Sangho Park, Wook Kim, Joonghyuk Kim, Jong Soo Kim, Yongsik Jung, Soohwan Sul, Soo-Ghang Ihn, Changho Noh, Sunghan Kim and Youngmin You “Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers”, Nat. Commun. 2018 vol. 9, no. 1, p. 1211.
Excellent blue luminescent materials, in particular high efficient blue phosphorescent material molecules having both a stable structure and a suitable luminescence spectrum, are particularly scarce. The lowest triplet excited state energy of the blue phosphors is much higher than that of the red and green phosphors, which means that the lowest triplet excited state energy of a host material of the blue devices should be even higher. Therefore, the organic structural units that can achieve the design of the blue luminescent range are limited, and thus it is more difficult to adjust the appropriate blue light spectrum. In addition, it needs to be able to exhibit excellent performance in the light-emitting process of the device. These require some specific and fundamental designs of structure.
Blue light is in a range of 400-500 nm (i.e. in a range of 400-495 nm or 400-490 nm). However, it is generally believed that blue light having short wavelength in a range of 400-450 nm (high energy blue light) is most harmful to eyes, can cause digital visual fatigue and affect sleep, and eventually cause eye pathological damages, such as myopia, cataracts, and macular degeneration, and break human rhythm. By designing a blue light source in a range of 450-500 nm and applying it to related electronic products, the problem of damages caused by the high-energy blue light in the current electronic devices can be solved.
Blue luminescent materials are used in OLED devices and apparatus as a dopant material, and their molecules participate in luminescence by stimulated emission of radiation as zero-dimensional points, which is the most demanding material in all OLED device structures in terms of stability. The literature, Wook Song and Jun Yeob Lee “Degradation Mechanism and Lifetime Improvement Strategy for Blue Phosphorescent Organic Light-Emitting Diodes” Adv. Optical Mater. 2017, briefly reports that the current stable blue light iridium complexes material having desirable stability, but none of the luminescent materials succeeded in controlling the majority of photoluminescence spectrum within the range of 450-490 nm. There remains a problem that emission color purity does not meet the requirement or that a large part of the light goes beyond the range of 450-490 nm, which needs to be filtered off, causing complicated process and waste of energy.
In terms of emission color purity, the tetradentate ligand-coordinated platinum complexes have the advantage that the energy level of spectral electronic vibration can be regulated, thereby achieving the goal of narrowing the spectrum and improving the color purity. Specific theoretical explanations can be found in reference Cong You, Fang Xia, Yue Zhao, Yin Zhang, Yongjian Sheng, Yipei Wu, Xiao-Chun Hang*, Fei Chen, Huili Ma, Kang Shen, Zhengyi Sun, Takahiro Ueba, Satoshi Kera, Cong Zhang, Honghai Zhang, Zhi-Kuan Chen, Wei Huang Probing Triplet Excited States and Managing Blue Light Emission of Neutral Tetradentate Platinum (II) Complexes 2018 Journal of Physical Chemistry Letter. Specific examples of applications of theory can be found in the patent application No. CN201810115217.5 (2018), titled Aryl-Substituted Tetradentate Ligand Coordinated Platinum Complexes, Synthesis Method And Use thereof, by Hang Xiaochun, Xia Fang, Sheng Yongjian, Qin Tianshi, Huang Wei.
In term of stability of metal complexes, the tetradentate ligand coordinated platinum compounds that have a given structure and contains a similar functional group are more stable than bidentate ligand coordinated iridium complexes, which is consistent with the conventional mechanism that higher chelating degree results in higher stability. An specific explanation may be that the iridium complex Ir(pmi)3 containing phenylimidazole ligand is not stable and will gradually decomposes when placed in the air at room temperature. The literature Tissa Sajoto, Peter I. Djurovich, Arnold Tamayo, Muhammed Yousufuddin, Robert Bau and Mark E. Thompson Ir(pmi)3 Blue and Near-UV Phosphorescence from Iridium Complexes with Cyclometalated Pyrazolyl or N-Heterocyclic Carbene Ligands Inorganic Chemistry 2005, 44, 7992-8003 describes a tetradentate ligand coordinated platinum complex PtON7, which also contains a phenylimidazole structure (pmi)NHC coordination, can exist stably. Moreover, high purity compounds can be prepared through purification by high temperature sublimation. Furthermore, associated devices have very high external quantum efficiency (EQE) and blue light luminescence spectra. The literature Xiao-Chun Hang, Tyler Fleetham, Eric Turner, Jason Brooks and Jian Li Highly Efficient Blue-Emitting Cyclometalated Platinum(II) Complexes by Judicious Molecular Design Angew. Chem. Int. Ed. 2013, 52, 6753-6756 demonstrates that the tetradentate ligand platinum complexes, whose structures are specially designed, are more stable than the iridium complexes and are more suitable to be used as a blue light dopant material in photoelectric devices.
In the NHC-coordinated blue light material, Ir(pmi)3 is unstable. Enlarging the aromatic system of the NHC ring can improve the stability. For example, the iridium complex Ir(pmb)3 with 2-phenylbenzoimidazole (pmb) used as its ligand has desirable stability, and the molecule itself is somewhat stable and will not decompose and change in a short time. Tissa Sajoto, Peter I. Djurovich, Arnold B. Tamayo, Jonas Oxgaard, William A. Goddard and Mark E. Thompson Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes J. Am. Chem. Soc. 2009, 131, 9813-9822. The stability of the structure design is further improved, the 2-phenylpyridinoimidazole (pmp) coordinated iridium complexes Ir(pmp)3 can exist stably, and the OLED blue light devices produced by using it provide a brightness greater than 22000 cd/m2, literature: Jaesang Leel, Hsiao-Fan Chen, Thilini Batagoda, Caleb Coburn, Peter I. Djurovich, Mark E. Thompson and Stephen R. Forrest Deep blue phosphorescent organic light-emitting diodes with very high brightness and efficiency 2015 Nature Materials. This demonstrates that application of the structure unit pmp in dopant material and blue light dopant material provides improved device efficiency and stability.
Photoelectric devices using organic materials are becoming more and more popular, and there are many reasons for their popularity. Many of the materials used to make such devices are relatively inexpensive, and thus organic optoelectronic devices have potential cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for special applications, such as processing on flexible substrates. Examples of organic photoelectric devices include organic light emitting devices (OLEDs), organic transistors, organic solar cells, and organic photoelectric detectors. For OLEDs, organic materials may have performance advantages over traditional materials. For example, the emission wavelength emitted by the organic light-emitting layer can usually be easily adjusted with a suitable dopant.
Light, i.e., fluorescence, is produced when the exciton experiences decay from the singlet excited state to the ground state. Cold light, i.e., phosphorescence, is produced when the exciton experiences decay from the triplet excited state to the ground state. Because the strong spin-orbit coupling of heavy metal atoms can very effectively enhance the intersystem crossing (ISC) between the singlet excited state and the triplet excited state, phosphorescent metal complexes, such as platinum complexes, have showed the potential to harvest both the singlet and the triplet excited states and to reach 100% internal quantum efficiency. Therefore, the phosphorescent metal complex is a good candidate for the dopant of the light-emitting layer in an organic light-emitting device (OLED) and has attracted much attention from both academic and industrial fields. Moreover, in the past decade, certain achievements have been made on the road to high-margin commercialization of this technology. For example, OLEDs have been applied in advanced displays for smartphones, televisions and digital cameras.
However, blue electroluminescent devices remains so far the most challenging area of the technology, at least in part due to the instability of blue devices. It is generally recognized that the choice of matrix material is a factor affecting the stability of the blue devices. However, the blue phosphorescent material has very high the lowest triplet excited state (T1) energy, this generally means that the lowest triplet excited state (T1) energy of the matrix material of the blue devices should be higher, bringing difficulties to the development of the matrix materials for the blue devices. Therefore, it is necessary to develop a blue luminescent material with superior performance.